We've invested heavily in improving site speed, and have seen a marked improvement in our speed as repored in Webmaster Tools' "SIte performance" over the past couple of months. We have reach up to "slower than 45% of sites".
However, looking at the result today we are "On average, pages in your site take 5.0 seconds to load (updated on Sep 13, 2011). This is slower than 74% of sites."
Testing some of the so-called slow loading pages with Page Speed reveals that many have very good scores, for example the homepage received 94/100.
We did recently add on the Facebook and +1 buttons, but this did not have an impact in the Page Speed result when tested. +1 is supposed to load asynchronously anyway.
I am a little perplexed as to the result in Webmaster Tools and was wondering if anyone could help explain this?
I would test your pages from some other corners of the globe. Therein lies the answer. Time to get that CDN and Anycast DNS. It's amazing how adding a button and milliseconds to a page tests just great near server home but it makes a hell of a difference to someone on another continent. Im pretty sure page speed is measured worldwide. Facebook definitely makes a difference in my tests, +1 not so much.